1. Field of the Invention
The present invention relates to a switching power supply, and more specifically, to a switching power supply that outputs a DC voltage of, for instance, DC 2V or lower.
2. Discussion of Background
Normally, a switching power supply converts DC to DC through switching performed at a DC--DC converter unit and outputs a DC voltage for a load through a pair of output lines. It is provided with an output voltage detection circuit constituting a means for achieving stabilization by monitoring the DC output voltage. The detection signal output by the output voltage detection circuit is provided to a control circuit. The control circuit implements output voltage stabilizing control on the DC--DC converter unit based upon the detection signal thus provided.
The output voltage detection circuit may assume a circuit structure comprising a resistive potential divider circuit, a 3-terminal regulator and a light emitting diode constituting a photocoupler, as disclosed in, for instance, Japanese Unexamined Patent Publication No. 242581/1996. The resistive potential divider circuit, which is connected between a pair of output lines, divides a DC output voltage and inputs a voltage resulting from the division to the control terminal of the 3-terminal regulator. The light emitting diode is connected in series to the main circuit of the 3-terminal regulator. The serial circuit thus achieved is connected between output lines of the DC--DC converter unit.
When the DC output voltage rises, thereby raising the divided voltage as well, the 3-terminal regulator performs an operation to increase/decrease the current flowing through the main circuit. This causes the current flowing through the light emitting diode connected in series to the 3-terminal regulator to increase/decrease accordingly.
The control circuit receives the detection signal via a phototransistor which, together with the light emitting diode, constitutes the photocoupler and implements control on the DC--DC converter unit to stabilize the DC output voltage.
The switching power supply assuming the circuit structure described above operates in a stable manner in the DC output voltage range of several volts and higher, which has been the range of output voltages of most switching power supplys to date. However, the output voltage detection circuit in the prior art cannot support the increasingly lower voltages that are observed with increasing frequency in recent years in electronic circuits driven by this type of switching power supply.
Namely, since the serial circuit constituted of the 3-terminal regulator and the light emitting diode in the output voltage detection circuit is connected between the output lines of the DC--DC converter unit, the DC output voltage manifesting between the output lines must be higher than the sum (approximately 2.35V) of the operating voltage of the 3-terminal regulator (approximately 1.25V) and the forward voltage at the light emitting diode (approximately 1.1V). If the DC output voltage becomes lower than the voltage corresponding to the degree of voltage reduction (approximately DC 2.35V) required at the 3-terminal regulator and the light emitting diode, the output voltage detection circuit can no longer engage in stable operation. In other words, a reliable output voltage stabilizing control function cannot be assured in a switching power supply with an output voltage of DC 2V or lower by adopting the circuit structure of the prior art.